July 23, 2010 -- People who have hay fever and who also have an allergy to cats, dogs, dust mites, or grass pollen have hay fever symptoms that are more severe and occur earlier on, according to a new study.

Hay fever season occurs in late summer when ragweed is in full bloom. However, not everyone allergic to ragweed experiences symptoms at the same time or in the same way. An estimated 36 million Americans have seasonal allergies. Ragweed is a plant that can grow anywhere; it is common throughout the Northeast, but it also grows in the South and Midwest and is a major cause of late-summer and fall allergy symptoms. People allergic to ragweed experience itchy eyes, runny noses, and sneezing.

Allergy Study Results

To evaluate who develops hay fever symptoms and when, researchers from Kingston General Hospital in Ontario, Canada, studied 123 people who were allergic to ragweed. The participants underwent skin prick tests to determine who was also allergic to mixed grasses, mixed trees, certain molds, pets, pollen, and dust mites. Two-thirds of the group tested positive for cat allergies, 63% tested positive for dog allergies, and 73% tested positive for dust mite allergies. Participants were then exposed to ragweed for three hours and asked to record their symptoms every 30 minutes.

"On average, those who tested positive for cat, dog, or dust mite allergies developed symptoms either faster than, or to a greater degree than those who tested negative for those allergies," says allergist and study researcher Anne K. Ellis, MD. "The differences seen at 90 minutes of exposure were less dramatic after three hours of exposure, however. That suggests that once the hay fever season is in full swing, the symptom differences between those with cat, dog, or dust mite allergies and those without no longer exist."

The findings suggest that year-round allergies to dogs, cats, and dust mites might pre-prime the immune system, increasing a person's sensitivity to other allergens, such as ragweed. The results are published in a recent issue of Annals of Allergy, Asthma & Immunology.

The researchers suggest that intense reactions to ragweed might be reduced by limiting exposure to other allergens before ragweed season begins. Allergy immunizations may also be an option, they say, and may make hay fever season more tolerable.

SOURCES: News release, International Association of Physicians in AIDS Care.